1. Field of the Invention
The present invention relates to a method of forming a groove for a dustproof boot of a bottomed cylindrical piston formed with the dustproof boot at a vicinity of an opening end portion of the bottomed cylindrical piston formed by forging, and a piston manufactured by the method.
2. Related Art
FIG. 8 shows an example of a bottomed cylindrical piston 1 of a disc brake of a background art. The disc brake is constituted to exert a brake force by extending an opening end portion 1a of the bottomed cylindrical piston 1 from an end portion of a hydraulic cylinder 2 to be butted to a friction pad assembly 3 and frictionally engaging the friction pad assembly 3 with a brake disc 4 by the bottomed cylindrical piston 1. However, invasion of dust or the like is remarkable at a portion thereof extended from the hydraulic cylinder 2. Therefore, a dustproof boot 5 is attached to the hydraulic cylinder 2 and a vicinity of the opening end portion 1a of the bottomed cylindrical piston 1. In order to attach the dustproof boot 5 to the vicinity of the opening end portion 1a, it is necessary to make a diameter on a side of the opening end portion 1a smaller than that of a piston main body side 1b. Normally, the vicinity of the opening end portion 1a is formed into a small diameter portion by cutting by a bit tool. After forming the small diameter portion by cutting in this way, a groove 6 for the dustproof boot is formed by cutting again. Therefore, there poses a problem of ensuring a wall thickness in view of a strength of the small diameter portion in order to form the groove 6 of the dustproof boot. In addition, there is a difficulty in which not only cost is increased since twice cutting is required but also a long working time period is taken. Further, there also poses a problem of bringing about a machining swarf.
In order to avoid the method of forming the groove for the dustproof boot by cutting in this way, also a method by form rolling has already been developed (refer to, for example, JP-U-51-92540). FIG. 9(a) is an outline view for explaining of forming a groove by a form rolling roll, FIG. 9(b) is a sectional view of a bottomed cylindrical piston as a final product after forming the groove. As shown by FIG. 9 (a), at an opening end portion 7a of a bottomed cylindrical piston 7 formed from a steel block by forging, the bottomed cylindrical piston 7 is rotated while exerting an external force to a portion thereof slightly remote from an end face from an outer periphery thereof by a form rolling roll 8 to thereby form a recess portion 9 in a ring-like shape at an outer peripheral face of the bottomed cylindrical piston 7. At this occasion, a wall thickness in view of a strength thereof is ensured by restraining a reduction in a diameter of the opening end portion 7a by pressing a roll 10 also from a side of an inner peripheral face of the bottomed cylindrical piston 7. Further, as shown by FIG. 9(b), after forming the recess portion 9 in the ring-like shape, a recess groove 9a for attaching a dustproof boot is shaped by cutting the recess portion 9.
According to the method of forming the groove for the dustproof boot by form rolling of the background art described in JP-U-51-92540, although there is achieved an advantage of ensuring the wall thickness of the opening end portion of the bottomed cylindrical piston since cutting is not carried out in a procedure of forming the recess portion 9 in the ring-like shape, since the groove is fabricated by rotating the bottomed cylindrical piston 7, it takes a long working time period. In addition, after forming the recess portion 9, the recess groove 9a needs to be shaped by cutting, and the machining swarf is unavoidable.